1. Field of the Invention
This invention pertains to non-crosslinked linear low density polyethylene preexpanded particles and a manufacturing method therefor, and more particularly to preexpanded particles used in the expansion and formation of non-crosslinked polyethylene expansion-molded articles in bead molds, which articles are used as cushioning packaging materials in the packaging of home electric goods, components in general, and electronic equipment components, and to non-crosslinked linear low density polyethylene preexpanded particles wherewith molded articles can be provided that exhibit good moldability during said expansion molding, the external appearance of which is attractive, and which exhibit outstanding cushioning properties.
2. Description of the Related Art
Expansion-molded articles made in bead molds of non-crosslinked linear low density polyethylene preexpanded particles, because of the properties of the base resin, exhibit suitable softness and mechanical strength, and are therefore widely used as cushioning materials for packaging home electric goods, components in general, and electronic equipment components. Disclosures concerning such non-crosslinked linear low density polyethylene preexpanded particles are made, for example, in Unexamined Patent Application No. S62-15239/1987 and Unexamined Patent Application No. S58-76433/1983 (both in gazette). The non-crosslinked linear low density polyethylene preexpanded particles disclosed in these patent applications employ as the base resin a copolymer of ethylene and an .alpha.-olefin having from 4 to 10 carbons, this copolymer having a melting point of 115 to 130.degree. C., or, alternatively, above 115.degree. C., and a resin density of 0.915 to 0.940 g/cm.sup.3, or, alternatively, 0.915 to 0.950 g/cm.sup.3, and an MI (melt index, and so hereinafter) of 0.1 to 5 (g/10 minutes, and so hereinafter). Preexpanded particles exhibiting outstanding moldability are said to be obtainable therewith, using, as means for enhancing in-mold moldability, a method for regulating the mean cell diameter of the preexpanded particles within a range of 120 to 1200 .mu., and means for making the high-temperature peak energy of the two heat-absorption peaks appearing in the DSC curve obtained by measurement with a differential scanning calorimeter to be 5 J/g or higher.
However, in cases of preexpanded particles having as their base resin one exhibiting a resin density of 0.930 g/cm.sup.3 or higher and a melting point of 125.degree. C. or higher, even when that base resin is a linear low-density polyethylene, the molded articles exhibit deteriorating impact resistance and brittleness, and crystal fusion is sharp, wherefore, even if the heating temperature during resin particle expansion is constant, as the heating time becomes longer, the energy (J/g) at the high-temperature peak of the two heat-absorption peaks that appear in the DSC curve obtained by measurement with a differential scanning calorimeter increases, and a phenomenon is observed whereby the mean cell diameter of the preexpanded particles becomes increasingly minute over the course of expansion. For this reason, when such preexpanded particles are mixed and used in molding, the surface properties of the molded articles are poor, and the mean cell diameters of a cut surface inside the molded articles are uneven and the appearance is poor. These are problems which remain.
In view of the situation described in the foregoing, linear low-density polyethylene having a resin density of 0.930 g/cm.sup.3 or higher and a melting point of 125.degree. C. or higher is currently not used in practice as a base resin for preexpanded particles. Moreover, the linear low-density polyethylene suitable for expansion, having a density of 0.925 to 0.928 g/cm.sup.3 and an MI of 2 to 5 g/10 minutes is all but unavailable from American and European resin manufacturers, and hence in many cases are imported from Japan for use, giving rise to the problem of enormous shipping cost.